This invention relates to the type of extrusion coater disclosed in U.S. Pat. No. 3,854,441 to George C. Park of Dec. 17, 1974 and disclosed in U.S. Pat. No. 3,919,974 to Peter Herzog of Nov. 18, 1975. Such coaters are especially designed to handle hot melts which are highly viscous in the order of 100,000 centipoises and to coat relatively wide webs such as eighty six inches or the like. The web is advanced under predetermined lateral and longitudinal tension over the extrusion slot and the thickness of the coating is metered by the pressure of the coating supply pump.
One of the problems encountered in coating such wide widths with high viscosity coating has been the excessive wear on the lead off lip despite the fact that the lips have been specially treated by a costly process to prevent wear. Another problem encountered in any slot coating apparatus is the lodging of specks of foreign material on the lead-off lip, this causing a continuous streak in the coating applied to the web until it can be removed usually during down time which is expensive.
It has been proposed in U.S. Pat. No. 3,496,012 to Biorseth of Feb. 17, 1970 to provide an extrusion slot coating head with a feed aperture, a discharge aperture and a rotatable rod metering means. The rotatable rod has a wire wound surface and no yieldable press roll is used to form a rolling nip with the rod. The rod is actually a metering roll which can be rotated in either direction at various speeds by a motor to thereby thin or thicken the coating. The Biorseth apparatus has a coating head which creates a cavity, or fountain, of the coating material across which a web is moved and the excess coating in the fountain travels back to the supply while a layer adheres to the underside of the moving web in the thickness determined by the speed and direction of rotation of the metering rod.
In other coating devices of the prior art, a similar power driven metering roll, rotating at the downstream side of a fountain, or pool, of liquid coating has also been used to meter the thickness of coating picked up by an applicator roll, the applicator roll depositing the coating further along the path on a surface of a moving web. Exemplary of such coaters are U.S. Pat. No. 2,560,572 to Haywood of 1951 and U.S. Pat. No. 1,983,982 to Vinollenberg of 1934. As in the Biorseth device, no pressure roll, rolling nip, or pressure rolling nip is disclosed in these patents.
Endless webs carried by backing rolls through a fixed clearance nip with an applicator roll are taught in U.S. Pat. No. 3,345,377 to Gettel of 1966 and U.S. Pat. No. 3,533,833 to Takahashi of 1970 but both patents disclose doctor means in rear of the nip, the Gettel doctor means being a small diameter rod on the end of a blade.
The rotating rods disclosed in U.S. Pat. No. 2,946,307 to Warner of July 26, 1960, U.S. Pat. No. 3,461,837 to Dreher of Aug. 19, 1969, and in German Pat. No. 1,964,908 of Dec. 30, 1968, all are characterized by support mountings for the rod which provide a movable axis of rotation by means of flexible, yieldable housings and all are for doctoring purposes or to serve as longitudinal, fluid metering means as in the Biorseth patent above.